[Bug c++/110213] New: Bogus (as opposed to false positive) -Wdangling-reference warning

[Bug c++/110213] New: Bogus (as opposed to false positive) -Wdangling-reference warning

[boris at kolpackov dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110213

            Bug ID: 110213
           Summary: Bogus (as opposed to false positive)
                    -Wdangling-reference warning
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: boris at kolpackov dot net
  Target Milestone: ---

I have a call that, AFAICS, doesn't involve any binding of references to
temporaries but which nevertheless still produces this warning. The code boils
down to the following:

const target& search (const target&, name);

void pattern::apply(target& t) const
{
  name n = ...;
  const target& p (search (t, std::move (n))); // <- Warning points here.
}

However, I was not able to reduce it to a small reproducer so it seems to
depend on the overall context. So I am attaching a partially preprocessed TU.
To reproduce, run:

$ g++ -std=c++23 -Wall -Wextra -fdirectives-only -c adhoc-rule-regex-pattern.ii

adhoc-rule-regex-pattern.cxx: In member function ‘virtual void
build2::adhoc_rule_regex_pattern::apply_prerequisites(build2::action,
build2::target&, const scope&, build2::match_extra&) const’:
adhoc-rule-regex-pattern.cxx:485:21: warning: possibly dangling reference to a
temporary [-Wdangling-reference]
  485 |       const target& pt (search (t, move (n), *s, &e.type));
      |                     ^~
adhoc-rule-regex-pattern.cxx:485:32: note: the temporary was destroyed at the
end of the full expression 
‘build2::search((*(const build2::target*)(& t)), build2::name((* &
std::move<build2::name&>(n))), (* s), (&
e.build2::adhoc_rule_regex_pattern::element::type))’
  485 |       const target& pt (search (t, move (n), *s, &e.type));
      |                         ~~~~~~~^~

The last two arguments (*s, &e.type) in the call do not make any difference.
Changing the search() function to take name by const& or && makes the warning
disappear.

$ g++ --version
g++-13 (Debian 13.1.0-3) 13.1.0

[Bug c++/110213] Bogus (as opposed to false positive) -Wdangling-reference warning

[boris at kolpackov dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110213

--- Comment #1 from Boris Kolpackov <boris at kolpackov dot net> ---
Created attachment 55304
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55304&action=edit
reproducer

[Bug c++/110213] Bogus (as opposed to false positive) -Wdangling-reference warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110213

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase:
#include <utility>
struct f
{
  f(const f&);
  f();
};

struct g{};

g &search(f);

void h()
{
        f n;
        const g& pt (search (std::move(n)));
}

[Bug c++/110213] Bogus (as opposed to false positive) -Wdangling-reference warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110213

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't see an issue with this warning really as there is a temporary being
created for the argument of type name and that is what the issue is warning
about. the argument of type name is still passed via reference even and the
temp is created on caller side as needed by the C++ standard even.

There are other bugs dealing Wdangling-reference where they have false positive
warnings dealing with different types of being warned about.

[Bug c++/110213] Bogus (as opposed to false positive) -Wdangling-reference warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110213

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
         Resolution|---                         |WORKSFORME
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
You can now disable the warning by using the [[gnu::no_dangling]] attribute:

https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=c7607c4cf18986025430ca8626abfe56bfe87106